1. Field of the Invention
This invention relates generally to downhole tools, and more particularly to a jar for inflicting axial force to a downhole string.
2. Description of the Related Art
In oil and gas well operations, it is frequently necessary to inflict large axial blows to a tool or tool string that is positioned downhole. Examples of such circumstances are legion. One situation frequently encountered is the sticking of drilling or production equipment in a well bore to such a degree that it cannot be readily dislodged. Another circumstance involves the retrieval of a tool or string downhole that has been separated from its pipe or tubing string. The separation between the pipe or tubing and the stranded tool or "fish" may be the result of structural failure or a deliberate disconnection initiated from the surface.
Jars have been used in petroleum well operations for several decades to enable operators to deliver such axial blows to stuck or stranded tools and strings. There are a few basic types. So called "drilling jars" are frequently employed when either drilling or production equipment has become stuck to such a degree that it cannot be readily dislodged from the well bore. The drilling jar is normally placed in the pipe string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via a manipulation of the drill string. These impact blows to the drill string are intended to dislodge the stuck object and permit continued operation. So called "fishing jars" are inserted into the well bore to retrieve a stranded tool or fish. Fishing jars are provided with a mechanism that is designed to firmly grasp the fish so that the fishing jar and the fish may be lifted together from the well. Many fishing jars are also provided with the capability to deliver axial blows to the fish to facilitate retrieval.
Jars capable of inflicting axial blows contain a sliding joint which allows a relative axial movement between an inner mandrel and an outer housing without allowing relative rotational movement therebetween. The mandrel typically has a hammer formed thereon, while the housing includes an anvil positioned adjacent to the mandrel hammer. Thus, by sliding the hammer and anvil together at high velocity, a substantial jarring force may be imparted to the stuck drill string, which is often sufficient to jar the drill string free. For most fishing applications it is desirable that the drilling jar be capable of providing both an upward and a downward jarring force.
There are four basic forms of jars: purely hydraulic jars, purely mechanical jars, bumper jars, and mechanical-hydraulic jars. The bumper jar is used primarily to provide a downward jarring force. The bumper jar ordinarily contains a splined joint with sufficient axial travel to allow the pipe to be lifted and dropped, causing the impact surfaces inside the bumper jar to come together to deliver a downward jarring force to the string.
Mechanical, hydraulic, and mechanical-hydraulic jars differ from the bumper jar in that they contain some type of triggering mechanism which retards the motion of the impact surfaces relative to each other until an axial strain, either tensile or compressive, has been applied to the drill string pipe. To provide an upward jarring force, the drill pipe is stretched by an axial tensile load applied at the surface. This tensile force is resisted by the triggering mechanism of the jar long enough to allow the pipe to stretch and store potential energy. When the jar triggers, this stored energy is converted to kinetic energy causing the impact surfaces of the jar to move together at a high velocity. To provide a downward jarring force, the pipe weight is slacked off at the surface and, if necessary, additional compressive force is applied, to put the pipe in compression. This compressive force is resisted by the triggering mechanism of the jar to allow the pipe to compress and store potential energy. When the jar triggers, the potential energy of the pipe compression and pipe weight is converted to kinetic energy causing the impact surfaces of the jar to come together at a high velocity.
The triggering mechanism in most mechanical jars consists of some type of friction sleeve coupled to the mandrel which resists movement of the mandrel until the load on the mandrel exceeds a preselected amount (i.e., the triggering load). The triggering mechanism in most hydraulic jars consists of one or more pistons which pressurize fluid in a chamber in response to movement by the mandrel. The compressed fluid resists movement of the mandrel. The pressurized fluid is ordinarily allowed to bleed off at a preselected rate. As the fluid bleeds off, the piston translates, eventually reaching a point in the jar where the chamber seal is opened, and the compressed fluid is allowed to rush out, freeing the mandrel to move rapidly.
Mechanical jars and hydraulic jars each have certain advantages over the other. Mechanical jars are generally less versatile and reliable than hydraulic jars. Many mechanical jars require the triggering load to be selected and preset at the surface to trigger at one specific load after the jar is inserted into the well bore. If it is necessary to re-adjust the triggering load, the jar must be pulled from the well bore. Other mechanical jars require a torque to be applied to the drill string from the surface in order to trigger the jar. The applied torque to the drill string not only represents a hazard to rig personnel, but torque cannot be applied to coiled tubing drill strings. Another significant disadvantage of mechanical jars is apparent in circumstances where the jar must be placed in a cocked position prior to insertion into the well bore. Thus, in those circumstances, the triggering mechanism is subjected to stresses during the normal course of if the jar is run as part of the bottom hole assembly. Finally, many mechanical jars have many surfaces that are subject to wear.
Hydraulic jars offer several advantages over purely mechanical jars. Hydraulic jars have the significant advantage of offering a wide variety of possible triggering loads. In the typical double acting hydraulic jar, the range of possible triggering loads is a function of the amount of axial strain applied by stretching or compressing the drill pipe, and is limited only by the structural limits of the jar and the seals therein. In addition, hydraulic jars are ordinarily less susceptible to wear and, therefore, will ordinarily function longer than a mechanical jar under the same operating conditions.
However, hydraulic jars also have certain disadvantages. For example, most purely hydraulic double acting jars are relatively long, in some instances having a length exceeding 25 feet. The length of a particular jar is ordinarily not a significant issue in drilling situations where regular threaded drill pipe is utilized. However, in coiled tubing applications, it is desirable that the length of all the tools in a particular drill string be no longer than the length of the lubricator of the particular coiled tubing injector. Thus, it is desirable that the jar be as short as possible to enable the operator to place as many different types of tools in the drill string as possible while still keeping the overall length of the drill string less than the length of the lubricator. A conventional hydraulic jar may take up one-half or more of the total length of a given lubricator, thus leaving perhaps less than half the length of the lubricator to accommodate other tools such as a mud motor, an orienting device, or a logging tool.
Many hydraulic jar designs also have a disadvantageously long metering stroke. The metering stroke is the amount of relative movement between the mandrel and the housing that must occur for the jar to trigger after it is cocked by application of an axial load. When an ordinary hydraulic jar is cocked by application of an axial load, fluid is pressurized in a chamber to resist relative movement of the mandrel and the housing. One or more metering orifices in the jar allow the compressed fluid to bleed off at a relatively slow rate. As the fluid is bleeding off, there is some relative axial movement between the mandrel and the housing. The amount of relative axial movement between the mandrel and the housing that occurs after the jar is cocked, but before the jar triggers, is known as bleed off. The bleed off represents lost potential energy that would ordinarily be converted into additional jarring force. Many current hydraulic jar designs have a relatively long metering stroke of 12 inches or more and, therefore, a significant amount of bleed off. A long metering stroke also leads to heat buildup in the hydraulic fluid, which may require costly intervals between firings and lead to degradation of fluid.
Mechanical-hydraulic jars ordinarily combine some features of both purely mechanical and purely hydraulic jars. For example, one design utilizes both a slowly metered fluid and a mechanical spring element to resist relative axial movement of the mandrel and the housing. This design has the same disadvantages associated with ordinary hydraulic jars, namely length, long metering stroke, and fluid heating. Another design utilizes a combination of a slowly metered fluid and a mechanical brake to retard the relative movement between the mandrel and the housing. In this design, drilling mud is used as the hydraulic medium. Therefore, the string must be pressurized before the jar will operate. This pressurization step will ordinarily require a work stoppage and the insertion of a ball into the work string to act as a sealing device. After the jar is triggered, the ball must be retrieved before normal operations can continue.
Some conventional jars employ a collet as a triggering mechanism. The collet is provided with one or more radially projecting flanges or teeth which engage a mating set of projections or channels in the mandrel. The engagement of the collet teeth and the mandrel teeth or channels restrains the longitudinal movement of the mandrel until some desired trigger point is reached. The trigger point frequently corresponds to the vertical alignment between the collet teeth and a channel or set of channels in the tool housing. At this point, the collet is no longer compressed radially inwardly and can expand rapidly in diameter to release the mandrel. The surfaces of the collet teeth and the channel or channels of the housing engaged just triggering may be subject to significant point loading, which can lead to rapid wear and the need for frequent repair. Furthermore, some conventional designs do not provide structure to prevent the premature expansion of the collet, which can otherwise lead to a sticking of the mandrel or a premature triggering. Premature triggering can lead to diminished overpull and application of less than desired axial force.
Many well operations are presently carried out with strings that utilize electrical power. Such tool strings are often suspended from conducting and non-conducting cables, such as wirelines and slicklines. In some wireline and slickline operations, it may be desirable to deploy ajar with tool string. If the jar is incapable of transmitting electrical power and signals, it must be positioned in the bottom hole assembly ("BHA") below the electrically powered components of the BHA. However, this may not be the optimum position for the jar in view of the operation to be performed.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.